1. Field of the Invention
The invention relates to multi-band antennas. More specifically, the invention relates to a multi-band antenna having a low profile, for example, suitable for mounting on a motor vehicle.
2. Description of Related Art
Modern vehicles may have several different radio receivers and or transmitters operating in different frequency bands. Previously, each band required its own separate antenna structure, or dual band antennas where available for two or three bands, for example the AMPS, UMTS and PCS cellular telephone frequency bands. Multiple bands may be serviced by discrete antenna structure, arranged in a common antenna housing to reduce costs by requiring only a single protective antenna enclosure and vehicle mounting point/hole for routing cabling for interconnection with the vehicle wire harness leading to the different receivers/transmitters.
Satellite Digital Audio Radio (SDAR) is a form of digital satellite radio, currently offered on a subscription basis by XM(trademark) and Sirius(trademark). SDAR receives in the S-Band frequency range (2.3 Gigahertz Band) requiring upper hemisphere coverage. To provide reception in urban environments where satellite line of sight signals may be blocked by earth contours, buildings and/or vegetation SDAR uses both satellite and terrestrial mounted transmitters and therefore requires antennas with vertical radiation patterns (satellite) as well as improved low angle performance (terrestrial). XM(trademark) specifies antenna performance of 2 dBic over a range of 25-60 degrees elevation. Sirius(trademark) specifies antenna performance of 3 dBic over 25-75 degrees elevation and 2 dBic over 75-90 degrees elevation.
Growth of SDAR, and GPS adds a potential requirement for two or more additional antennas. Rather than mounting several discrete antennas on a vehicle, vehicle manufacturers and consumers prefer multi-band antenna assembles with a minimized vertical profile. Low profile antennas increase resistance to accidental breakage from, for example, automated car washes and tree limbs. Less visually noticeable from a distance, low profile antennas also reduce vandalism and theft opportunities. Also, negative effects on aerodynamics and disruption of vehicle design aesthetics are minimized.
Competition within the antenna industry has focused attention on minimization of antenna materials and manufacturing costs.
Prior SDAR antennas have used a left hand circularly polarized quadrifilar antenna element configuration. Another antenna element configuration used with SDAR is the curved cross dipole configuration. Both types of antenna structures have antenna element vertical heights of at least one inch.
Circular microstrip antennas have a fundamental TM11 excitation mode with a narrow beam. Circular microstrip antennas have been used for satellite reception where an upper hemisphere radiation pattern with poor low angle coverage is acceptable, for example with Global Positioning Satellites (GPS). Circular microstrip antenna designs are inexpensive, durable and have an extremely low profile. Microstrip antennas may be configured to operate in a TM21 higher order mode that creates a conical radiation pattern with a null at center/vertical, useful for receiving low angle terrestrial originated signals.
Hula-Loop (directional-discontinuity ring-radiator) antennas comprising a looped conductor with a feed and a ground leg are a known solution for low profile antennas for AMPS and GSM cellular radio frequencies. However, this antenna configuration has previously been usable only for a single band and the resulting ring form had a large diameter compared to other known AMPS/GSM band antenna configurations, for example low profile monopoles.
Therefore, it is an object of the invention to provide an antenna, which overcomes deficiencies in the prior art.